This invention pertains generally to the field of microfluidics and to ultrasonic actuators.
Microfluidic devices have potential application in many areas, including the production and analysis of pharmaceuticals, in medical diagnoses, and in drug delivery. A particular problem encountered in devices having microfluidic channels is that the analyte, for example, latex beads with antibodies thereon, is dispersed at low densities along the channel at low Reynold""s number flow. The low density results in a low signal to noise ratio in the detected signal, for example, the fluorescent signal from the latex beads. It would be desirable to be able to concentrate the beads in the microfluidic channels to enhance the potential signal to noise ratio.
For applications such as the concentration of analytes as discussed above, and for pumping fluids in microfluidic channels, acoustic streaming has several advantages compared to other techniques. For example, the stress waves responsible for acoustic streaming can be excited far away from the channel, eliminating the need to integrate electrodes in close proximity to the channel. Other pumping or mixing methods such as electro-osmosis, electro-hydrodynamic pumping, magneto-hydrodynamic pumping, and electrophoretic pumping require that the liquid be electrically conductive. In contrast, acoustic streaming based ultrasonic pumps or mixers are far less dependent on or sensitive to the electrical or chemical properties of the fluid. Thermal and piezoelectric bimorph pumps are based on large mechanical displacement of the fluid. See, e.g., P. Gravesen, et al., xe2x80x9cMicrofluidicsxe2x80x94a Review,xe2x80x9d J. of Micromechanics and Microengineering, Vol. 3, No. 4, December 1993, pp. 168-182. In contrast, in acoustic streaming, the displacements are very small (on the order of nanometers), but the high frequencies used result in high particle velocities. Various micro systems have been reported which utilize acoustic streaming. See, e.g., R. Zengerle, et al., xe2x80x9cMicrofluidics, xe2x80x9d Proc. of the Seventh International Symposium on Micro Machine and Human Science, 1996, pp. 13-20; A. Lal, et al., xe2x80x9cUltrasonically Driven Silicon Atomizer and Pump, xe2x80x9d Solid State and Actuator Workshop, Hilton Head Island, USA., Jun. 3-6, 1996, pp. 276-279; H. Wang, et al., xe2x80x9cEjection Characteristics of Micromachined Acoustic-Wave Ejector,xe2x80x9d The 10th International Conference on Solid-State Sensors and Actuators, Sendai, Japan, Jun. 7-10, 1999, pp. 1784-1787; P. Luginbuhl, et al., xe2x80x9cFlexural-Plate-Wave Actuators Based on PZT Thin Film, xe2x80x9d Proc. of the 10th Annual International Workshop on Micro Electro Mechanical Systems, Nagoya, Japan, Jan. 26-30, 1997, pp. 327-332; R. M. Moroney, et al., xe2x80x9cMicrotransport Induced by Ultrasonic Lamb Waves, xe2x80x9d Vol. 59, No. 7, August 1991, pp. 774-776; X. Zhu, et al., xe2x80x9cMicrofluidic Motion Generation with Loosely-Focused Acoustic Waves, xe2x80x9d The 9th International Conference on Solid-State Sensors and Actuators, Chicago, Ill., Jun. 16-19, 1997, pp. 837-838. A common feature of these investigations is the use of bulk micromachined SiN membranes or bulk silicon which is excited by piezoelectric thin films.
In accordance with the invention, selective pumping, guiding, mixing, blocking, and diverting of fluids in microcavities in micromechanical systems can be carried out simply and efficiently without requiring mechanical or electrical connections to elements within the microcavities. Further, particles within the fluid in the cavities, such a microspheres, can be concentrated or dispersed, as desired, for purposes such as enhancement of detection of signals from the particles or for filtering purposes. The control of fluid motion within the microcavities is carried out utilizing microstructures in the cavities having cantilever elements that are coupled to a substrate to receive vibrations therefrom. The cantilever elements can be excited into resonance at one or more resonant frequencies. By selection of the shape of the cantilever elements, their position in the microcavity, the spacing of the cantilever elements from the walls of the cavity, and the frequency at which the cantilever elements are excited, the direction of pumping of fluid through the cavity can be controlled, blocked, or diverted.
Exemplary microfluidic actuation apparatus in accordance with the invention includes a substrate, structural material on the substrate defining a cavity having a bottom wall, sidewalls, and a top wall, and an ultrasonic actuator in the cavity having a cantilever element projecting into the cavity that is spaced from the bottom wall and the top wall of the cavity. The cantilever element is coupled to the substrate to receive vibrations therefrom and has a resonant mode of vibration at a resonant frequency. An ultrasonic vibrator is coupled to the substrate outside of the cavity to transmit ultrasonic vibrations to the substrate and from the substrate to the cantilever element. The ultrasonic vibrator may comprise, for example, a high frequency driver such as a piezoelectric plate that is capable of vibrating at various frequencies from about 100 KHz to 10 MHz. Depending on the frequency of vibration applied to the substrate and thus to the actuators, the cantilever elements of the actuators may provide acoustic streaming of fluid in the cavity to pump fluid, as through a channel from one port to another, or may create vortices adjacent to the vibrating elements that trap or control the flow of fluid.
The ultrasonic actuators may be formed with various structures, including cantilever elements formed as plates extending outwardly from a pedestal fixed to the substrate and cantilever plates extending outwardly from a sidewall of the cavity.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.